The end of the world?

Predictions that the world will end on 21 December 2012 are based on a misinterpretation of why the Mayan long-count calendar doesn’t go past that date. But what are some of the real ways the world could end?

The kinetic energy of a crashing asteroid is converted into heat, light and sound – creating pressure waves that destroy anything within a given radius. And the blast will kick up dust into the atmosphere, blocking out sunlight and preventing photosynthesis in plants and removing a vital part of the food chain.

It’s likely that an asteroid of at least 1 km in diameter will be on a collision course with Earth within the next 500,000 years – but impacts can be averted. Despite several films depicting the destruction of such an asteroid using a nuclear device detonated below its surface, it’s probably more efficient to simply deflect it by a few degrees, taking it out of the Earth’s path.

Nuclear winterIn the event of a large-scale use of nuclear weapons, not only would dangerous radioactive material be spread across the globe, but, just as in the case of an asteroid impact, dust and soot would be kicked up into the atmosphere, blocking out sunlight.

The Sun’s natural evolutionThe distance between a planet and a star at which water is liquid, and therefore life is possible, is colloquially known as the “Goldilocks zone” – not too hot, not too cold, but just right.

But if asteroid impacts or leaders a bit too itchy-fingered on the big red button don’t end up blocking out so much light that the Earth is too cold, the Sun will eventually become too hot.

As the nuclear fusion reactions in its core use up more and more hydrogen, the total number of particles in the Sun decreases and it contracts in size. Some of the gravitational potential energy released makes the Sun heat up, increasing the rate of nuclear reactions and, with it, the Sun’s luminosity.

In about 600 million years’ time, the power output from the Sun will be large enough to affect Earth’s geology, making rocks trap more carbon dioxide. Eventually there’ll be so little of it in the atmosphere that plants’ photosynthesis will be impossible and all multicellular life will become extinct.

In about a billion years, the Sun’s luminosity will be 10% higher than now, and Earth will be so hot that the oceans will evaporate.

Another four or five billion years later and the Sun will have exhausted the supply of hydrogen at its core, and will begin its evolution into a red giant. Its outer layers will expand over the subsequent three billion years until the star is 256 times its present size – engulfing the rocky inner planets.

Man-made climate changeThe planet could possibly heat up much sooner if fossil-fuel burning goes on unchecked.

The “greenhouse effect” predicts that some of the solar radiation received by the Earth is trapped by the atmosphere. It’s this that regulates the Earth’s temperature and keeps it at a level we find comfortable, compared to the Moon’s fluctuating between -153C at night and 107C during the day.

Some substances trap more heat than others, and it’s also thought that the higher concentrations of these greenhouse gases in the Earth’s early history kept it at a temperature at which liquid water was possible despite the Sun only being 70% as bright as it is today.

But one of these greenhouses gases is carbon dioxide, a byproduct of burning fossil fuels in power stations or vehicles. It’s thought that adding more and more of it to the atmosphere will cause more and more heat to be trapped, meaning the average temperature of the Earth will rise.

TechnologyIf we succeed in creating artificial intelligence, the computers mightturn outto be mean. However, even assuming that thinking machines are possible they’re probably some way off, and can be built with safeguards – and you’d have to be pretty dim to give one the keys to the bomb.

That hasn’t stopped scientists considering the problem – Cambridge University’s Centre for the Study of Existential Risk has begun to evaluate the potential threats posed by renegade robots.

Another apocalyptic scenario in which technology is culpable is that of “grey goo” –nanotechnology that self-replicates and, in doing so, consumes all the resources on Earth. Even Prince Charles is worried by the idea. Being overrun by trillions of nanobots is unlikely however, since, as expert Eric Drexler points out, building self-replicating nanoscale machines in the first place is redundant given their great complexity and inefficiency.

Alien invasionA popular sci-fi scenario, but is an alien takeover really plausible?

Traversing the vast distances between stars is a difficult business, and a resource-intensive one – it seems unlikely that any civilisation capable of managing it would need to plunder the Earth. However, Stephen Hawking has warned against advertising our presence to anyone who may be listening out for us, citing mankind’s less than illustrious history when a more advanced civilisation makes first contact with a more primitive one.

The lack of evidence for the existence of other intelligent species when the galaxy should be abundant with them makes it appear less likely that humanity will ultimately get through the other threats to its survival unscathed.